Random numbers form sequences of numbers or symbols that lack any pattern and appear random. A random number generator (RNG) is a computational or physical device designed to generate random numbers. RNGs can be classified in pseudo-RNGs (PRNGs), computational algorithms, and true-RNGs (TRNGs). TRNGs are physical devices designed to generate sequences of numbers or symbols that lack any pattern. Moreover, RNGs implemented with physical devices can be subdivided into classical RNGs (CRNGs), based on classical hardware devices with unpredictable behaviour, and quantum RNGs (QRNGs) based on quantum effects [1, 2, 3].
Current commercial RNG devices are based on: quantum single photon detector arrays [4], CMOS metastability, noise signal by using the stochastic physical phenomenon of electrons trapped in the silicon nitride layer of a transistor, arrival detection time of photons of a continuous wave (cw) operated laser, reversed bias semiconductor junction, thermal or Johnson noise and transistor noise. Several documents describe those devices: a light beam illuminating a quantum detector array [5], wave diffraction using a high-order grating [6], photon detection as random events [7], photon coupling to a single-mode coupler [8], electrical noise [9, 10], single photon laser beam splitting using homodyne detection [11].
The publication “High-speed quantum random number generation by measuring phase noise of a single-mode laser” by Bing Qi et. al. [12] discloses a QRNG based on measuring the quantum phase noise of a single-mode semiconductor laser. The phase noise of the laser originates from amplified spontaneous emission (ASE) when the laser is operated very close to its threshold. The system has a 500 Mbit/s random number generation rate, limited by the capability of the system to enlarge the ASE bandwidth to reduce the coherence time. A phase modulator is used to reduce the impact of periodic drifts that limits the length of the generated random sequence. However, besides being an additional element, the phase modulator is itself intrinsically subjected to drift, if electro-optics materials such as LiNbO3 are used. In addition, the fact that the laser is operated very close to its threshold makes it more difficult to avoid classical noise, which reduces the level of quantum randomness associated to ASE.
There is thus a need for a higher rate QRNG source, which shortens the coherence time, avoids the use of a phase modulator and where the impact of classical noise is strongly reduced.